I was playing Terraria for a while yesterday until I thought, why not make an SCP based from one of the bosses? I chose Plantera, because it's easy to adapt and make your own. If there are similar ideas please tell me.

Object Class: Euclid

My idea is of a large plant bulb that is capable of transforming any non-organic matter that enters it into toxic spiky balls. these balls bounce around and are coated with an unknown venom that is lethal. These balls are produced anyways, though at a much slower rate than when inorganic matter is ingested. However, any organic matter will be ingested as food. Too little food will cause it to enter an enraged state and possibly breach containment, though too much will cause it to expand in size and begin growing tentacles that function like the Sarlacc in Star Wars. These tentacles can last years, even if food consumption is at the optimal amount. need critique. Thanks!

Off the bat, don't say "enraged state". It's on a guide of things not to do, it's not a thing that actually exists. I feel like this suffers from "a thing that kills you" and "generic monster". While taking inspiration from a monster that already exists is fine, you need to do way more with it. I feel like you just described what the monster does in the game, but tacked on some stuff to make it slightly different.

I can't say its interesting based off your description. It seems like something that could just be neutralized instead of contained. Why would the Foundation want to keep it alive? Will you be able to explain the biology and workings of this creature in believable detail?

I think if you really want to run with this idea, you need to take a lot more time to develop it. SCPs are catalysts for a story. It has to evoke an emotion and not just be a dangerous creature. On a basic level it's sounds like a cool monster. But it's not a special, unique monster at this stage.